The goal
Let's tackle your end goal rather than your proposed approach. We want to get an aux input into the car's on-board installed audio receiver.
Mixer
You can just mix the two audio channels (CD player and iPhone). Honestly, this is most likely what you really want and you might find that you like this better than switching between sources.
The mixer will give you both audio sources all the time. Want to listen to just iPhone, press pause on the CD changer, want only the CD changer, don't play music on the phone.
The cool hidden feature here is that iPhone notifications (for example when you are using navigation, when there are incoming messages, when using SIRI, etc...) will play on top of your other source so you can get the periodic output from your phone without having to not listen to your CD's.
Mixer's broadly come in two types passive and active. For your application, I would strongly suggest an active mixer, since you will likely find the input levels too low with a passive solution (therefore you will have to turn your receivers' volume up very high, which is dangerous and bad for the speakers).
A simple, and perfectly viable approach is this one (following), which uses a couple of resistors, capacitors, and a single op-amp. It works on the principle of superposition, which I will explain in more detail if you so desire.
You can either go with adjustable resistors as suggested in the schematic or play with it on your test bench and then use fixed values for the final design. I'd recommend that approach, since cars vibrate and mechanically adjusted resistors tend to lose their position and tolerance over time under such conditions.
This circuit, as drawn, only mixes a single channel. For car audio, you typically have a stereo signal so you will need to duplicate the circuit (one for left and one for right). You mix, as you might expect, the two left channels together and the two right channels together resulting to two independent outputs (your mixed stereo signal).
Switching approaches
I suspect that you will use a mixer, but if you truly want switching here are some alternatives.
Microphone port impedance detection
I don't own an iPhone so I can't test it explicitly, but one way to do this is make use of the input impedance of the microphone port. The iPhones detect the presence of an external microphone by it's impedance. They test the input impedance of the microphone ring. You can detect their detecting and switch on that signal.
Audio tap
This circuit looks like the mixer above, but with only one input and you aren't going to listen to the output.
Use a very small capacitor (say 1nF) to tap the audio line and feed it into a high-impedance, high-gain configuration of an op-amp. It will be horrible audio, but you aren't going to listen to it and it will be easily distinguishable from white-noise. You can threshold the output and switch on that.
Put a mechanical switch
Why do we need to switch electronically? How about putting a SPDT switch in your glove box or on your dashboard.
Put a Wi-Fi connected switch
This guy made a Wi-Fi connected relay that he controls from his iPhone. There are commercial products available as well.
